A straight line is 180°. So you can do:
(15x - 4) + (5x - 8) = 180 Simplify
20x - 12 = 180
20x = 192 Find the value of x
x = 9.6
m∠ABD = 15x - 4 Plug in x = 9.6
m∠ABD = 15(9.6) - 4 = 144 - 4 = 140°
m∠DBC = 5x - 8 Plug in 9.6
m∠DBC = 5(9.6) - 8 = 48 - 8 = 40°
